Yup. As a VERY conservative start, I'd at least block the area that's not directly in front a radiator. But the block, if not done in front of the grille, would have to go directly up against its "back" on the inside. But that'll just trap bugs 'n' stuff, won't it?
Any air you can keep from going through the grille and ultimately exiting under the car is better off going over/around the outside.
Just as a reality / perspective check: I measured just under a 3% increase in FE by agressively blocking my grille openings (which has a MUCH larger stock open area and smaller radiator= more room for improvement, than the Yaris).